1. Field of the Invention
The present invention relates to a lamp for a backlight, and particularly, to an external electrode fluorescent lamp of a backlight which is appropriate for a large-area liquid crystal display device.
2. Discussion of the Related Art
With the recent development of portable electronic devices, such as mobile phones, personal digital assistants (PDAs), and notebook computers, the demand for light weight, thin, and small flat panel display devices used in portable electronic devices is increasing. Research includes active development of flat panel display devices including liquid crystal display (LCD) devices, plasma display panel (PDP) devices, field emission display (FED) devices, and vacuum fluorescent display (VFD) devices. Of these different devices, LCD devices are actively being developed due to the simple mass-production techniques available to produce them, the ease of driving them, and high picture quality.
Such LCD devices are a transmission type display devices, and they display a desired image on a screen by controlling the amount of light transmitted through a liquid crystal layer by refraction anisotropy of liquid crystal molecules. Accordingly, a backlight (a light source transmitting through the liquid crystal layer to display an image) is installed in the liquid crystal display device. In general, backlights are divided into two types.
The first is an edge-type backlight in which lamps are installed at the side of a liquid crystal display (LCD) panel and supply light to the liquid crystal layer. The second is a direct-type backlight in which lamps installed under the LCD panel directly supply light thereto.
The edge-type backlight is installed at the side of the LCD panel and may supply light to the liquid crystal layer through a reflector and a light guide plate. Thus, the thickness of the LCD panel may be reduced, and therefore, the edge-type backlight is mainly used in the notebooks requiring thin display devices. However, because the edge-type backlight has lamps located at the side of the LCD panel, it is hard to use the edge-type backlight in a large LCD panel and to obtain high brightness because light is supplied through the light guide side. Accordingly, the edge-type backlight is not appropriate for an LCD panel for a large LCD televisions which have recently increased in popularity.
The direct-type backlight may be used in a large LCD panel and produce high brightness levels because light emitted from the lamps is supplied directly to the liquid crystal layer. Thus, the direct-type backlight is mainly used to fabricate LCD panels for LCD televisions.
FIG. 1 is a view schematically illustrating the construction of a liquid crystal display device to which a direct-type backlight is applied in accordance with the related art. As illustrated therein, a liquid crystal display (LCD) device 1 includes a liquid crystal display (LCD) panel 3 and a backlight 10 installed at the back of the LCD panel 3. It is the LCD panel 3 where an image is displayed. The LCD panel 3 includes a transparent lower substrate 3a, a transparent upper substrate 3b, such as glass, and a liquid crystal layer (not illustrated) formed therebetween. In particular, though not illustrated in the drawing, the lower substrate 3a is a TFT substrate on which a driving device such as a thin film transistor and a pixel electrode are formed, and the upper substrate 3b is a color filter substrate on which a color filter layer is formed. In addition, a drive circuit unit 5 is on the side of the lower substrate 3a and supplies signals to the thin film transistor and the pixel electrode that are formed on the lower substrate 3a. 
The backlight 10 includes a plurality of lamps 11 for supplying light to the LCD panel 3, a reflector 17 reflecting the light emitted from the lamps 11 and improving light efficiency, and an optical sheet 15 diffusing the light emitted from the lamps 11 and making the diffused light incident upon the LCD panel 3.
The direct-type backlight will be described in detail with reference to FIG. 2. As illustrated in FIG. 2, the direct-type backlight 10 includes a lower case 21, a reflector 17 above the lower case 21 reflecting light emitted from the lamps 11, a lamp fixing unit 24 installed above the reflector 17 and fixing the lamps 11, a side supporting unit 26 installed at both sides of the lamp fixing unit 24 and supporting the lamp fixing unit 24, a diffusion plate 15a and a plurality of diffusion sheets 15b and 15c above the lamps 11 diffusing the light emitted from the lamps 11 and supplying uniform light to the LCD panel, and an upper case 28 installed above the diffusion sheets 15b and 15c. 
In general, a cold cathode fluorescent lamp (CCFL) is used as the lamp, but the CCFL is operated at high brightness of about 30,000 cd/m2, and the life span of the lamps is short. A CCFL may be used in not only a direct-type backlights but also the edge-type backlights. However, when the CCFL is used in the edge-type backlight, because brightness is low, it is not appropriate for a panel for a large-screen. In addition, when the CCFL is used in the direct-type backlight, because the an interval between the lamps needs to be enlarged in order to maintain proper brightness of the LCD panel, a reflector having a special structure is required and also the distance between the diffusion plate and the lamps becomes great to thereby increase the LCD panel in thickness.
Accordingly, there remains a need for a backlight that has a long life span, high brightness level, and high efficiency for a large-screen LCD device. An external electrode fluorescent lamp (EEFL) has been developed to meet such a demand.
The EEFL has external electrodes at both ends and is turned on by supplying continuous high-frequency voltages or high-frequency voltages in the form of pulses to the external electrodes. Such an EEFL is illustrated in FIG. 3.
As illustrated in FIG. 3, light is emitted from the EEFL when both ends of a fluorescent lamp 11 are inserted into cap-shaped external electrodes 21 and fluorescent materials emit light by electric discharge inside the fluorescent lamp 11 according to high-frequency voltages being supplied to the EEFL.
As the LCD panel increases in size, the fluorescent lamp 11 increases in length. For LCD panels of more than 30 to 40″, the length of the lamp will exceed 700 mm. As the lamp increases in length, brightness becomes non-uniform, and when turning on the lamp, a high lamp voltage is required. Thus, stability problems occur and image quality is deteriorates according to electrical effects of a driving circuit of the LCD panel.